Which organism is most commonly associated with intra-abdominal infections among anaerobic gram-negative rods?

• A. Bacteroides fragilis
• B. Bacteroides melaninogenicus
• C. Fusobacterium nucleatum
• D. Prevotella intermedia

Answer: (A)

Intra-abdominal infections are typically polymicrobial, with a strong contribution from organisms that reside in the gut. Among anaerobic gram-negative rods, Bacteroides fragilis is the most commonly involved. It lives in the colon and, when the intestinal wall is breached, it can invade the peritoneal cavity and form abscesses. Its virulence factors, including a polysaccharide capsule that helps it resist the immune response and its tendency to produce beta-lactamases, make it a frequent and clinically significant finding in these infections. The other organisms listed are more commonly associated with oral infections or head-and-neck contexts; they can appear in broader polymicrobial infections but are not the primary anaerobic gram-negative rod linked to intra-abdominal disease.